The present invention relates generally to the field of optical fiber communications systems and particularly to the measurement and compensation of optical component characteristics such as polarization mode dispersion (PMD) in such components and in systems that comprise such components. More particularly, the invention relates to a measurement apparatus and method that uses entangled photon technology.
As the demand for increased bandwidth continues, telecommunications providers are looking for new ways to provide the additional bandwidth. The ultimate bandwidth available in an optical communications channel (e.g., an optical fiber) is limited by its optical properties. In particular, if the transmission time of an optical pulse through an optical channel is dependent on its polarization, the channel is said to exhibit polarization mode dispersion. PMD results from the birefringence of optical materials in the optical path which leads to a differential propagation delay between the orthogonal polarization components of light transmitted through the optical channel. PMD limits the bandwidth of the optical channel because it broadens the optical pulses and increases the bit error rate (BER). As modulation speeds increase, pulse durations decrease, and accurate compensation of PMD are required to maintain a low BER. To control such compensators, precise characterization of the PMD of the optical channel is required.
Prior art methods of characterizing PMD have depended on classical optical (as opposed to quantum optical) phenomena. For example, the NetTest NEXUS Polarization Mode Dispersion Measurement System employs a Michelson interferometric technique to analyze PMD. Essentially these prior art systems attempt to measure the amplitude and relative phase of the two vector components of the polarized light. Other prior art systems use an optical signal analyzer (OSA) to measure the effects of PMD (that is, the system measures power variations at a fixed set of output polarization states as function of wavelength). In the former case, the light that has passed through the device under test must be divided into two arms of an interferometer, potentially introducing non-common path errors, while with the second approach the dispersive phase delay is not measured directly, it being inferred from the measured intensity variations.
One measurement apparatus that overcomes these difficulties was described in U.S. patent application Ser. No. 10/147,149. That apparatus uses quantum-optical phenomena to measure the effective time delay (the PMD) between polarization states of light the have propagated through an optical component, including an optical communications fiber. In that apparatus, pairs of quantum-optically entangled photons, each pair being a “twinon”, are propagated through the optical component under test. At the output port of the optical component the twinons are directed into a two arm, quantum interferometer that includes a variable polarization-dependent delay element in one arm. The variable polarization dependent delay is scanned until a maximum in quantum coherence is detected. The delay inserted by the variable delay element when the correlation is maximum is then deemed to be the PMD of the element.
If an optical communications signal is simultaneously propagating through the element under test, the measured delay can be applied to separate a pre- or post-element compensation device, such compensation eliminating the detrimental effects of the delay.
The measurement apparatus (and method of measurement) itself and the functional separation between measurement and compensation suggested by the '149 application has several operational limitations. First, in order to identify the maximum in quantum coherence, the apparatus must scan through the maximum point. Second, in order to track the maximum (as is required when the PMD is time varying), the apparatus must continually rescan (or dither) in a range centered on the previous maximum point. Since said scanning and dithering is time consuming, there is a significant delay between the measurement of the PMD and the application of a compensating delay in the signal channel. That is, the compensation bandwidth is limited by the scan and process time of the measurement process. Thirdly, the functional separation between the measurement and the compensation requires a calibration to link the two; that is, a user must insure that the actual compensation applied accurately corresponds to the valued measured.
It is therefore desirable to have a measurement apparatus and method in which, once the PMD is determined, the PMD can be tracked without the need for dithering; such a method is typically based on a “signed” error signal, which indicates the direction in which a correction is required, and seeks a null in that error signal. Additionally, it is desirable to embody the compensation apparatus into the measurement apparatus such that the action of nulling the error signal in the measurement inherently adjusts the compensation apparatus to the proper value.
It will be appreciated that although PMD is used throughout as an example, the measurement and compensation method of the present invention applies equally to other optical characteristics such as chromatic dispersion, optical activity, or index change with temperature.